Vacuum-assisted countergravity casting processes using gas permeable investment shell molds are described in the Chandley U.S. Pat. Nos. 3,900,064; 4,340.108; 4,532,976; 4,589,466 and 4,791,977.
In the fabrication of the gas permeable, high temperature bonded refractory investment shell molds for use in such countergravity casting processes, a plurality of expendable (e.g., meltable) patterns of the article to be cast are first formed and then assembled with suitable ingate patterns and the like to form a pattern assembly or tree. The pattern assembly is then invested with refractory particulate by alternately dipping the pattern assembly in a refractory slurry (comprising refractory powder and a suitable binder solution capable of hardening during drying under ambient conditions) and then dusted or stuccoed with coarser refractory powder. The sequence of dipping and stuccoing is repeated to build up a multi-layered refractory shell having a sufficient thickness to resist stresses imparted thereto by subsequent pattern removal, firing and metal casting operations.
In particular, the pattern removal operation typically has been carried out by steam autoclaving wherein the invested pattern assembly is placed in a steam autoclave heated to a temperature in the range of about 275.degree. F. to about 350.degree. F. to melt out the pattern from the refractory shell. In the past, prior art workers have experienced damage (e.g., cracking) to the refractory shell during the steam autoclaving step as a result of thermal expansion of the pattern (e.g., wax) relative to the refractory shell. In efforts to reduce or minimize damage (e.g., cracking) of the refractory shell during the steam autoclaving step, prior art workers have increased the thickness of the shell to better withstand these stresses. Unfortunately, increasing the thickness of the refractory shell results in heavier investment shell molds and consumption of greater quantities of refractory material, adding to the cost of casting. Moreover, the greater thickness of the refractory shell also requires conduct of the steam autoclaving operation for longer times to effect removal of the pattern from the invested pattern assembly. Typically, investment shell molds used to countergravity cast iron base and other alloys using the aforementioned patented processes are made to have a shell wall thickness of at least about 1/4 inch to this end.
Aforementioned U.S. Pat. No. 4,791,977 describes stresses imposed on the refractory shell mold during the vacuum-assisted countergravity casting of molten metal therein. In particular, that patent recognizes that harmful stresses can be imposed on the shell as a result of internal metallostatic pressure exerted thereon by the metal cast therein in conjunction with the external vacuum applied about the shell mold during the casting process. The patent recognizes that such stresses when combined with the high temperatures of the metal in the shell mold can cause shell wall movement, metal penetration into the walls, metal leakage and outright failure of the shell mold, especially if there are any structural defects in the shell. Although this patent provides a means of reducing such stresses on the investment shell mold (i.e., by using a differential pressure technique between the internal mold fill passage and vacuum chamber external of the shell), the investment shell mold used in that patent is still required to have a conventional shell wall thickness and strength to withstand stresses during pattern removal and molten metal casting.
It is an object of the present invention to provide an improved, economical countergravity casting method and apparatus that uses a refractory investment shell mold having a significantly reduced wall thickness that nevertheless is less subject to damage (e.g., cracking) during operations such as pattern removal by steam autoclaving.
It is another object of the present invention to provide an improved, economical countergravity casting method and apparatus which significantly reduces the amount of bonded refractory material needed to fabricate the investment shell mold.
It is still another object of the invention to provide an improved, economical countergravity casting method and apparatus which significantly increases the number of castings that can be cast per investment shell mold.
It is still a further object of the invention to provide an improved countergravity casting method and apparatus which reduces stresses imposed on the investment shell mold by the presence of internal metallostatic pressure and external vacuum conditions about the shell during casting.
It is still a further object of the invention to provide an improved countergravity casting method and apparatus which supports the investment shell mold during casting in such a manner as to prevent damage to the mold from casting stresses, permit larger molds to be cast and to prevent molten metal leakage therefrom.